The desirability of maintaining correct inflation pressure in automobile tires and the like is well established. Improper tire inflation has been determined to decrease fuel economy, increase tire wear, and decrease safety. Optimum fuel economy results when automobile tires are inflated to the proper pressure where the rolling resistance of the tire is minimized. Over-inflation causes excessive wear near the middle of a tire's tread as the tire bulges outward. Conversely, under-inflation results in excessive wear at the edges of a tire's tread as the tire flattens. The United States National Highway Traffic Administration reported that 1.5 billion dollars in tire wear could be saved annually by proper tire inflation.
Safety is enhanced by proper tire inflation due to the improved control afforded thereby. The United States Highway Traffic Safety Administration reported that 260,000 accidents, 28,000 injuries, and 720 deaths were caused by improper tire inflation in a single year.
Although proper tire inflation may be maintained by regularly checking tire pressure and adjusting it accordingly, this procedure tends to be largely ignored because of the inconvenience involved. Furthermore, tire pressure varies with the temperature of the air in the tire and is consequently effected by vehicle speed, road surface, and ambient temperature.
Improved traction may be achieved by varying the inflation pressure depending upon the surface over which the vehicle is to be driven. Driving a vehicle over a hard surface, i.e. a paved road, is more efficiently accomplished with tires having a higher inflation pressure than is required to drive the same vehicle over a softer surface. Increased pressure is desirable when driving over a hard surface because it results in relatively low tire surface area contacting the road surface and consequently results in reduced tire wear and improved rolling resistance. However, greater traction is generally required when traveling over softer surfaces, i.e. sand or snow. Increased tire traction may be achieved by reducing the tire's inflation pressure. Such reduced inflation pressure results in an increase in the surface area of the tire in contact with the driving surface, thus enhancing traction.
Variations in tire inflation pressure also result from changes in the payload or weight carried by the vehicle. Increasing the payload increases the force exerted upon the tires. This necessitates a corresponding increase in tire pressure in order to maintain the optimum contact area of the tire upon the driving surface.
Systems for automatically inflating and deflating vehicle tires are well known. Typical among such systems are those which allow an operator to vary the pressure within a vehicle's tires as desired. Such systems are disclosed by U.S. Pat. No. 2,634,783, issued to Turek; U.S. Pat. No. 2,989,999 issued to Holbrook; U.S. Pat. No. 3,276,502 issued to Ruf; and U.S. Pat. No. 4,418,737 issued to Goodell et al. In each of these prior art tire inflation systems, air is provided from an air source located within the vehicle to the rotating tires via the use of rotary pneumatic joints and seals.
The use of such rotary seals involves several inherent disadvantages. Foremost, is the complexity of the installation, which increases both the cost and the potential for maintenance problems. Additionally, such rotary joints are necessarily operated in an environment which is inherently hostile to their performance. Various contaminants such as oil, water, dirt, and sand are almost continually present and thus represent a potential hazard to the integrity of the seal. The hostility of the environment is further aggravated by potential use of the vehicle upon unpaved surfaces whereby the vehicle is additionally subjected to rigorous shock and vibration.
Thus, although such automatic tire inflation systems are technically feasible for their intended purposes, they possess inherent economic and practical deficiencies which detract from their overall effectiveness in the marketplace.